1. Field of the Invention
The present invention relates to an improved archery bow and more specifically to a bow which provides for additional energy to be stored in, the limbs during the draw, making it more comfortable for the archer and capable of launching arrows at higher velocity.
2. Description of Related Art
Conventional recurve bows and longbows have limbs with flattened cross sections which experience bending around only a single axis as the bow is drawn. Their force-draw characteristics are approximately linear, with most bows exhibiting a gentle inverted xe2x80x9cSxe2x80x9d shaped curve with an ever increasing slope, or xe2x80x9cstackingxe2x80x9d as fill draw is approached. Upon release of the string, the arrow experiences an initial large force which rapidly drops off. The arrow shaft responds by bending and going through a series of oscillations as it leaves the bow. The complex dynamics of the arrow makes for an inefficient transfer of energy to the arrow, resulting in relatively slow flight speeds.
The compound bow makes a drastic alteration of the force-draw curve such that the draw force at full draw is lower than the peak draw force earlier in the draw. The action upon release of the string is a more appropriate and gradual delivery of force to the arrow, with the result that the arrow bends less and flies truer and faster, and the efficiency of energy delivery to the arrow is much improved over the conventional bow. Compound bows achieve their altered force-draw curves at the expense of much additional complexity and weight, and do not resemble the traditional archery bows.
Attempts have been made to construct bows with more traditional appearance having some of the advantages of compound bows, for example, U.S. Pat. No. 4,088,111 (Li) and U.S. Pat. No. 4,122,821 (Mamo) both attempt to store additional energy or modify the force-draw curves of the bows using a controlled deformation of the cross section of the limbs during the draw. The Li patent describes that additional energy can be stored during the draw by using the lateral deformation of elements of the cross section of the limb. The designs are effectively limited to lateral movements only, even going so far as including a hinge element to isolate that particular motion and eliminate other modes such as twisting. Li discloses storing energy by laterally stretching an elastic membrane and also by compressing trapped gas. The Li designs never made an impact on the traditional archery bow field as they are unwieldy.
Mamo discloses modifying the force-draw curve by using laterally curved rectangular cross section limbs which tend to increase their radius of curvature or flatten as the bow is drawn. In practice, if such cross sections are thick enough to provide sufficient bending stiffness for the bow, then they are too thick to flatten appreciably without exceeding the strength of the material from which they are made.
Both the Mamo and the Li patents indicate that their bows can have a draw force at full draw that is lower than the peak draw force earlier in the draw. However, the resulting negative draw force slope will always, result in an unstable bow, whereby one of the limbs xe2x80x9cbreaks overxe2x80x9d before the other during the draw. As with the compound bows, this instability can be overcome by mechanically coupling the limbs to synchronize their movements. Unfortunately, the resulting bow would have an appearance similar to that of a compound bow.
Further attempts to improve the performance of conventional bows have been tried by incorporating additional elements to the construction of the bow. Such attempts include extra limbs or other separate energy storage elements incorporated into the bow. These approaches, of course, represent steps away from the elegant simplicity of the traditional bow and are, in reality, alternate forms of compound bows. Examples of bows with additional elements added include: U.S. Pat. No. 5,454,361 (Bronnert); U.S. Pat. No. 3,674,001 (Hitt); U.S. Pat. No. 4,244,345 (Simo); U.S. Pat. No. 4,644,928 (Studanski); U.S. Pat. No. 4,041,927 (Van House); U.S. Pat. No. 4,207,859 (Scholten).
Bronnert describes a xe2x80x9csequential bowxe2x80x9d with a second bending member in contact with the main limb. Hitt describes adding an auxiliary limb to modify the behavior of the bow. Simo et al. describes a bow with a torsion bar element separate from the limbs that is twisted by contact with a camming device connected to the limbs, which are designed to be non-bending and simply rotate around a pivot at the handle. The Simo system, while it does store energy in torsion, results in a slow and inefficient bow because of the large movements required of the limbs. Van House, similar to Simo, involves the addition of a compression spring, which is also connected to the main limbs through a mechanical linkage. Studanski proposes using a secondary buckling member that spans from limb to limb, while Scholten provides buckling members mechanically linked to rotating main limbs.
While the advantages of altering the force-draw curve are known in the art, the majority approach the challenge by adding extra parts to the bow. Additionally, a few have thought of ways to achieve the wanted alteration by using some deformation of the cross section of the limb during the draw. However, none have recognized the substantial advantages of storing energy in the limbs by torsion or rotation of parts of the limb.
The present invention is a bow having a pair of limbs, where a portion of each limb is divided into two parallel sections. The parallel sections are initially twisted away from the flat orientation in a symmetric manner, so that the sections are twisted into a high-moment-of-inertia configuration. As the bow is drawn, the sections are increasingly twisted toward the flat, low moment-of-inertia configuration by the radial stresses that develop in the limb members during bending. In conventional bows, energy is stored in the limbs during the draw in the form of elastic strain energy due to bending. In the present invention, the energy due to bending is stored as usual, but the twisting of the parallel sections causes additional energy to be stored in the form of torsional elastic strain energy. This torsional elastic strain energy due to twisting can be made to develop mainly during the early and middle parts of the draw. Before each section twists to the flat, low moment of inertia configuration, those sections of the limb behave as stiffer, high moment of inertia sections. The bow thus behaves as a high draw force bow early in the draw and as a low draw force bow late in the draw after all sections have become flattened. The difference in energy under the force-draw curves of the invention bow and an equivalent flat-limbed bow is directly attributable to the torsional strain energy stored.
It is an objective of the present invention to provide an archery bow having limbs which store energy more abundantly and efficiently, thus resulting in a faster and truer arrow flight.
It is a further objective of the present invention to provide an archery bow that has the same stored energy as a conventional bow with a reduced full draw weight.
These and other objects, features and advantages of the present invention will be more readily understood with reference to the following detailed description, read in conjunction with the accompanying drawing figures.
All patents, patent applications and publications referred to or cited herein are incorporated by reference in their entirety to the extent they are not inconsistent with the explicit teachings of this specification, including; U.S. Pat. No. 4,088,111 (Li), U.S. Pat. No. 4,122,821 (Mamo), U.S. Pat. No. 5,454,361 (Bronnert), U.S. Pat. No. 3,674,001 (Hitt), U.S. Pat. No. 4,244,345 (Simo), U.S. Pat. No. 4,644,928 (Studanski), U.S. Pat. No. 4,041,927 (Van House), and U.S. Pat. No. 4,207,859 (Scholten).